Typical vane pumps known in the art include, e.g., the one illustrated in FIG. 10. This vane pump 1 has a rotor chamber 2 and a rotor 3 eccentrically accommodated in the rotor chamber 2. A plurality of vane grooves 19 is radially formed in the rotor 3 and vanes 4 are slidably moved in the respective vane grooves 19. Each of the vanes 4 is free to move in a radial direction of the rotor 3. As the rotor 3 is rotatably driven, the leading ends of the respective vanes 4 make sliding contact with the inner peripheral surface 2a of the rotor chamber 2, whereby working compartments 5 surrounded by inner surfaces of the rotor chamber 2, an outer peripheral surface 3a of the rotor 3 and the vanes 4 undergo a volume change and a working fluid drawn into the working compartments 5 from an inlet port 6 is discharged through an outlet port 7. For example, Japanese Patent Laid-open Application No. S62-291488 discloses the same vane pump as the one shown in FIG. 10.
It is possible for the vane pump shown in FIG. 10 to pressurize the working fluid in the working compartments 5 by the vanes 4 protruded from the outer peripheral surface 3a of the rotor 3. Since, however, portions of the outer peripheral surface 3a of the rotor 3 between adjacent vanes 4 have a smooth circular arc shape without unevenness, it is difficult to make strong-enough fluid flow in the working compartments 5. Accordingly, the fluid pressure in the working compartments 5 may not be high enough, resulting in the pump performance which can be further improved.